Lithium-ion secondary batteries with high energy densities have been used as power sources for portable electronics such as a mobile phone and a notebook computer.
The lithium ion secondary battery is structured with a positive electrode material, a separator, and a negative electrode material. As the positive electrode material, an aluminum alloy foil having superior electrical conductivity and less heat generation without affecting electrical efficiency of a secondary battery has been used. The positive material can be obtained by coating a paste onto both sides of the aluminum alloy foil, the paste containing a lithium-containing metal oxide such as LiCoO2 as a chief component, followed by drying of the coated paste, and then performing compression forming with a pressing machine (hereinafter this process of compression forming is referred to as press working). The positive electrode material thus prepared is then stacked with a separator and a negative electrode material. The stacked structure is then wound, shaped, and encased.
The aluminum alloy foil used for the positive electrode material of the lithium ion secondary battery requires high tensile strength, when the issues of cuts occurring due to the tension during the coating of the active material paste, and the ruptures occurring at the bending portion during winding are taken into consideration. In addition, in the conventional drying step carried out after the coating of the active material paste, heat treatment is carried out at 100 to 160° C. In recent years, there are cases where the heat treatment is carried out at higher temperatures of around 200° C. Here, with respect to the following press working carried out in order to increase the density of the active material, since the strength of the aluminum alloy foil after heat treatment is generally lower than that of the bare foil, high tensile strength after the drying step is also required. When the strength is lowered after the drying step, center buckle is likely to occur during the press working. This would induce wrinkles during winding, which reduces adhesion between the active material and the aluminum alloy foil. In addition, ruptures during the slitting process would likely occur, which would be a serious problem in the manufacture of batteries. In particular, when the adhesion between the active material paste and the surface of the aluminum alloy foil decreases, peeling off of the active material would proceed during the repeated charge and discharge of the battery. This would lead to a problem of decrease in the battery capacity.
Recently, thinning is required for the aluminum alloy foil used in the positive electrode material of the lithium ion secondary battery. Capacity increase and size reduction are seen in the lithium ion secondary batteries, and thus studies to thin the aluminum alloy film used for the positive electrode material in order to increase the capacity of the battery per unit volume have been made.
On the other hand, during the thinning of the aluminum alloy foil, too high tensile strength would result in lower rollability, which would be problematic.
Accordingly, regarding the aluminum ally foil used for the positive electrode material of the lithium ion battery, there have been a demand for thinner foil for higher battery capacity, higher bare foil strength for preventing cuts during the active material paste coating process, and higher strength after the drying step for preventing wrinkles during the press working. In these views, optimization of the conditions without impairing the rollability has been in demand.
In Patent Literature 1, an aluminum alloy foil for an electrode of a lithium ion battery, of which tensile strength of the bare foil being 240 MPa or more, has been suggested. In Patent Literature 2, an aluminum alloy foil for battery electrode current collector, of which tensile strength of the bare foil being 280 to 350 MPa and having superior rollability, has been suggested. However, the aluminum alloy foils disclosed in Patent Literatures 1 and 2 cannot give a solution to all of the issues simultaneously. That is, they cannot simultaneously solve the issues of thinning of the foil for higher capacity, preventing cuts during the active material paste coating process, the process involving a drying step at higher temperatures than the conventional ones, and preventing wrinkles during the press working.